Method for the production of finely divided metals



United States Patent 3,390,981 METHOD FOR THE PRODUCTION OF FINELYDIVIDED METALS Lewis C. Hoffman, Wilmington, Del., assignor to E. I. duPont de Nemours and Company, Wilmington, Del., a corporation of DelawareNo Drawing. Continuation-impart of application Ser. No. 258,607, Feb.14, 1963. This application Nov. 9, 1965, Ser. No. 507,038

17 Claims. (Cl. 75-108) This application is a continuation-in-part of mycopending application, Ser. No. 258,607, filed Feb. 14, 1963 nowabandoned.

This invention relates to the production of noble metal alloysconsisting essentially of two of the following metals: silver, gold,platinum and palladium. More particularly, it relates to the productionof such allows in finely divided form.

The noble metal alloys produced by this invention are palladium-gold,platinum-gold, silver-gold, silver-palladium, silver-platinum andpalladium-platinum alloys. Each of the above metal combinations formcontinuous series of solid solutions over the entire range of alloycompositions with no eutectics or compounds of the metals being formed.

Noble metal alloys in very finely divided form, e.g., an averageparticle size of no larger than 40 microns with no more than about 5% ofthe particles larger than 42 microns, and preferably with a particlesize not exceeding about 5 microns and most preferably of an averagesize in the range of 0.1 to 0.5 micron; are highly desired for theproduction of electric current conducting layers on dielectric surfaces,e.g., in the production of capacitors, resistors and conductors. Suchfinely divided noble metal alloys are virtually impossible to obtain bythe comminution of such alloys produced in massive form in the usualmanner, by melting together the metal constituents of the alloys.

It is therefore an object of this invention to provide a new method ofobtaining finely divided noble metal alloys consisting essentially oftwo of the metals; silver, gold, platinum and palladium.

It is another object of this invention to provide a method of obtainingfinely divided noble metal alloys consisting essentially of two of themetals; silver, gold, platinum or palladium, where the average particlesizes of thealloys are of the order of 0.1 to 0.5 micron.

Other objects will become apparent from the following detaileddescription of the invention.

The above objects may be accomplished, in general, by preparing asolution of the metal constituents of the alloy powder in a ratioapproximately equal to the compositions of the alloy desired, followedby simultaneously coprecipitating both dissolved metals to provide thedesired alloy through the use of a reducing agent or mixture of reducingagents. In the preparation of, for example, gold alloys devoid ofsilver, platinum alloys devoid of silver, and palladium alloys devoid ofsilver; chloride compounds (which are the most convenient form of thesemetals available) can be employed in the preparation of solutions fromwhich the alloys are precipitated. Other soluble compounds of thesemetals, e.g., cyanides, bromides, nitrates, etc. can be employed equallyas well. In the preparation of silver alloys, soluble salts of silver,such as the nitrates of this metal, obtained as such or formed bydissolving silver in nitric acid, can be employed. The other noble metalto be alloyed with silver must not, when placed in solution with thesilver,

3,390,981 Patented July 2, 1968 effect precipitation of the silver as aninsoluble salt. Accordingly, chlorides of noble metals are to be avoidedin making silver alloys and the nitrates and cyanides of these noblemetals can conveniently be employed.

In the preparation of solutions of palladium and silver, both the silverand palladium may be simultaneously dissolved in nitric acid or thepalladium may first be dissolved in the acid and the silver subsequentlydissolved therein. If desired, the given amount of palladium and silvermay be dissolved in separate quantities of nitric acid and the twosolutions mixed. Palladium will only dissolve in nitric acid containingat least 3% excess N0 and having a specific gravity of at least 1.5 g./cc. and, therefore, if the two metals are dissolved in a single quantityof acid, it will be most convenient to first dissolve the palladium andthen the silver in the nitric acid. Alternately, the palladium can bedissolved in nitric acid and crystalline silver nitrate in the properratio can be dissolved in the palladium nitrate solution.

The solvent employed with the noble metal compounds used to make thesolution from which the alloy powders are precipitated is convenientlywater. Other polar solvents can be used as desired. Chlorinatedhydrocarbon solvents can satisfactorily be used to dissolve noble metalhalide salts. Concentrations of the dissolved metals of from about 5 to25% by weight have proven satisfactory in this process, but allconcentrations of ingredients from the most dilute up to saturatedsolutions can be employed as desired. It has been observed that thefineness of alloy powders precipitated is influenced by theconcentrations of metals, with finer powders being produced from thelower concentrations. When employing Water as a solvent, the pH thereofshould be maintained acidic to avoid precipitation of metal noble basesand can be varied between pH values of from about 1 to 6. It has beenfound that the speed of precipitation increases with increasedtemperature and pH values. Generally, the coarseness of the alloy powderformed is directly proportional to the speed of precipitation. Highquality alloy powders have been obtained using the combination of roomtemperature and pH values of from 4.5 to 6.5

To produce the desired alloy, pairs of reducing agents, one of whichprecipitates one of the metal constituents from the solution and theother of which precipitates the other metal constituent, can be used incombination. In addition, a single reducing agent for both of the metalconstituents of the desired alloys can be used. It is only necessary informing the metal alloy that the metal constituents thereof besimultaneously precipitated from the same solution.

While, generally, any reducing agents for the alloy constituents whichcoprecipitate the alloy constituents and any soluble compounds of thealloy constituents can be employed, it will be appreciated that caremust be exercised to avoid competitive reactions including interreactionof the soluble compounds, undesired reactions of the metal compoundswith the reducing agents, and undesired interreactions between thereducing agents employed.

Hereinafter, are set forth examples of reducing agents and combinationsthereof which can be used in the process of the invention.

Hydrazine hydrate, H N:NH -H O, functions to reduce gold and palladiumfrom their solution to produce goldpalladium alloy powders and isequally usable with a goldplatinum solution to precipitate agold-platinum alloy powder.

Pairs of reducing agents may be selected from the following Table I toprecipitate palladium and gold alloy powders.

TABLE I For Au:

hydroquinone hydrazine sulfate sulfurous acid sodium sulfite zinc dustferrous sulfate Pairs of reducing agents may be selected from Table IIto precipitate platinum-gold alloy powder.

TABLE II For Au:

hydroquinone hydrazine sulfate sulfurous acid sodium sulfite zinc dustferrous sulfate It was found that the sodium borohydride liberateshydrogen in acid solution and that the metal powders formed therefromare saturated with hydrogen after filtration and drying. This hydrogenis evolved rapidly in the heating-up stage of the firing of electricaldevices which contain alloy powders made by this method with disruptiveconsequences in those instances where it is desired to produce fired-oncoatings having fine, or small dimensions. Accordingly, its use only forlarge cross-sectioned fired-on coatings is recommended. When employingsodium hydrosulfite and/or ferrous sulfate as reducing agents for themetals indicated above, the pH of the solution for which they are usedmust be adjusted to a value of from 4.5 to 6.5 or above. While hydrazinehydrate is the preferred reducing agent for the solutions ofpalladium-gold and platinum-gold, the pairs of reducing agents set forthabove also provide excellent results.

Hypophosphorous acid functions to reduce silver and palladium fromnitrate solutions thereof. Pairs of reducing agents may be selected fromthe following Table III to precipitate silver and palladium alloypowders.

TABLE III For Pd:

sodium borohydride hypophosphorous acid hydroquinone For Pd:

hypophosphorous acid sodium hydrosulfite For Pt:

sodium borohydride sodium hydrosulfite For Ag:

sodium formate ammonium formate hydroxylamine formic acid hydrazinesulfate tartaric acid The pH of the solutions may be adjusted to a pH ofbetween about 4.5 and 6.5 by the addition thereto of concentrated NH OH,NaOH, KOH, Na CO K Mg(OH) Ca(OI-I) Ba(OH) or the like. Of theseneutralizing agents, NH OH is particularly preferred especially when thesolution contains dissolved palladium, since it forms a complex with Pdand prevents the formation of small quantities of Pd(OH) The reductionof the nitrates and precipitation of the palladium-silver alloy fromsuch solutions produces an average particle size of alloy particlesslightly larger (probably 0.3-0.5 micron) than from nitrate solutions inwhich the pH has not first been adjusted to between 4.5 and 6.5. Thealloy particles produced :by reduction of pH- adjusted solutions are notquite as subject to shrinkage during firing for the production of anelectrically conducting film.

The neutralizing agents may be added as such or in concentrated aqueoussolution. The alloy powder containing silver and palladium produced whenammonium hydroxide is used as the neutralizing agent is superior forcertain uses such as capacitor or resistance electrodes to the powdersformed through the use of other neutralizing agents.

The following examples are given to illustrate, in detail, severalpreferred embodiments of the invention, it

being understood that the details of these examples are not to be takenas limitations of the invention.

EXAMPLE 1 310 grams of palladium sponge (commercial refiners grade) aredissolved in 5 liters of red, fuming nitric acid (Sp. gr. 1.53 g./cc.)at 5060 C. The solution is allowed to cool. 77.5 grams of silver arethen dissolved in the solution to give an /20 weight ratio of Pd/Ag.

With rapid stirring, one liter of 50% H PO solution is slowly droppedinto the above solution. A black precipitate forms and settles to thebottom of the reaction vessel. The precipitate is filtered off and driedto produce about 386.5 grams of alloy powder. This powder is found tohave an average particle size of about 0.3 micron. The particles, whensubjected to a gradual increase in temperature are found to have amelting point of about 1400 C., the known melting temperature of an80/20 Pd/Ag alloy as shown in the liquidus curve of Pd/Ag alloys.

By contrast, a mixture of finely divided Pd and Ag in the ratio of 80/20shows a melting point of 960 C., the melting point of silver, andsamples thereof heated to 1000 C., and cooled show the presence ofdiscrete large balls of silver in finely divided palladium.

When the precipitated alloy powder is shaken with a 7% aqueous solutionof nitric acid, the solution shows no presence of silver nitrate by theusual chloride test, whereas a mixture of finely divided Pd and Ag will,under the same circumstances, show the presence of silver nitrate.

EXAMPLE 2 310 grams of palladium sponge are dissolved in 5 liters ofred, fuming nitric acid (Sp. gr. 1.53 g./cc.) at 5060 C. The solution isallowed to cool. 77.5 grams of silver are then dissolved in the solutionto give an 80/20 weight ratio of Pd/Ag in the solution. 7.1 litersconcentrated ammonium hydroxide (Sp. gr. 0.9) are then slowly droppedinto the solution changing the solution from brown through red andyellow to yellow-green. The pH of the solution is about 5.5.

1 liter of 50% H PO solution is next slowly dropped into the abovesolution. A black precipitate forms and settles to the bottom of thereaction vessel. The precipitate is filtered oif and dried, yielding386.5 grams of alloy powder.

The powder has a melting point of about 1400 0., showing that it is analloy of Pd and Ag rather than a mixture of the two. The averageparticle size of this powder is about 0.4 micron.

EXAMPLE 3 Example 2 is repeated but using 1 liter of a 5050 mixture ofsodium formate and sodium borohydride, both 50% aqueous solutions, asthe reducing agent to reduce the palladium and silver nitrates to finelydivided metal alloy powder. The resulting precipitate, by the testsabove set forth in Examples 1 and 2, shows itself to be an alloy ofpalladium and silver and not a mixture of palladium particles and silverparticles. In a series of examples, using the procedure of Example 2,palladium and silver of proportions varying from 5 to parts Pd and to 10parts Ag were dissolved in nitric acid and reduced with H PO Theresulting alloy particles contained proportions of Pd and Agcorresponding substantially to their content in the nitric acidsolutions. These alloys had melting points corresponding with themelting points of Pd-Ag alloys on the known liquidus curve of Pd-Agalloys.

Examples of different materials and amounts thereof used to formplatinum-gold alloy powders are set forth below.

EXAMPLE 4 To obtain a 90% gold-10% platinum alloy, 24 grams of a PtCl,solution containing 32.67% platinum by analysis were mixed with 176grams of an AuCl solu- 5 tion containing 39.09% gold. The resultingsolution was diluted to 2500 ml. and a solution of 50 grams of hydrazinehydrate in 1000 m1. of deionized water was dropped in with rapidstirring. A black precipitate of the gold and platinum alloy formedwhich was allowed to settle, washed with water by decantation, filteredoff and dried. The observed melting point of this precipitate togetherwith the theoretical melting point of a 90% gold-% platinum alloy is setforth opposite Example 4 in Table IV.

Table IV also lists other platinum-gold alloys of various compositionswhich have been prepared by the same general method as set forth forExample 4, using the metal salt solutions of Example 4, but in differentrelative amounts and diluted to 2500 ml. The indicated reducing agentsdissolved in 1000 ml. of deionized water in the amounts shown were usedto precipitate the alloys. The observed melting points were obtained byplacing a thermocouple in a furnace containing the test sample of thealloy and observing the temperature at which the first liquid appearedas the temperature inside the furnace was increased. The theoreticalmelting points were obtained from the literature.

of palladium and gold. The resulting solution was diluted to 250 ml. anda solution of 4 grams of hydrazine hydrate in 100 ml. of deionized waterwas dropped in with rapid stirring. A black precipitate of thegold-palladium alloy formed which was allowed to settle, washed withwater by decantation, filtered off and dried.

The melting point of this alloy powder is set forth in the Table Vopposite Example 18. Table V also lists other Pd-Au alloys of variouscompositions which were prepared by the same general method as set forthby Example 18, using the PdCl and AuCl solutions of Example 18 but indifferent relative amounts. The solution resulting from mixing the PdCland AuCl solutions was in each case diluted to 250 ml. The reducingagents and the amounts thereof indicated in Table V dissolved in 100 ml.of deionized water were used to precipitate the alloys from thesolutions.

The observed melting points were obtained by placing a thermocouple in afurnace containing the test sample of the alloy and observing thetemperature at which the first liquid appeared as the temperature insidethe furnace TABLE IV Example Wt. of Pd Wt. of Au Percent ObservedMelting Number Solution Solution Pd in Reducing Agent and AmountDissolved in 100 Melting Point in gms. in gins. Alloy m1. of H20 Point,C. (theor.), C. 24 176 10 50 g. hydrazine hydrate l, 080 50 150 21. 4.do 1,129 68 132 1, 200 110 90 1, 330 130 70 1, 390 148 52 398 1, 425168 32 525 1, 550 184 16 do 700 1, 680 90 50 25 g. hydroquinone, 25 g.sodium borohydnde 1,332 1, 330 50 21. 4 25 g. hydrazine sulfate, 25 g.sodium borohydri 1,127 1, 129 110 90 50 25 g. sodium borohydride, 25 g.sulfurous ac 1,330 1, 330 110 90 50 25 g. sodium sulfite, 25 g. sodiumborohydride. 1, 337 1, 330 110 90 50 25 g. sodium borohydride, 25 g.zinc dust 1, 338 1, 330 110 90 50 25 g. sodium hydrosulfite, 25 g.ferrous sulfate 1, 341 1, 330

1 pH adjusted to about 5.5 with NaOH before adding reducing agent.

The observed melting point values demonstrate that the powder productsformed are true alloys since their was increased. The theoreticalmelting points were obtained from the literature.

TABLE V Example Wt. of Pd Wt. of Au Percent Observed Melting NumberSolution Solution Pd in Reducing Agent and Amount Dissolved in 100Melting Point in gms. in gms. Alloy ml. 011120 Pomt, C. (theor.), C.

15 85 10 4 g. hydrazine hydrat 1, 1, 200 27 73 20 do- 1, 360 1, 350 4060 30 1, 403 1, 400 50 50 40 454 1, 450 60 40 50 1, 490 1, 475 70 30 60A 1, 495 1, 500 77 23 70 1, 515 1, 520 87 13 80 1, 541 1, 540 93 7 90 o.1,570 1,550 60 40 50 2 g. hypophosphorous acid, 2 hydroquinone 1, 482 1,475 60 40 50 2 g. hypophosphorous acid, 2 g. hydrazine sulfate. 1, 4801, 475 60 40 50 2 g. hypophosphorous acid, 2 g. sulfurous acid 1, 490 1,475 60 40 50 2 g. hypophosphorous acid, 2 g. sodium sulfite 1, 481 1,47560 4o 50 2 g. hypophosphorous acid, 2 g. zinc dust 1, 476 1, 475 60 4050 2 g. sodium hydrosulfite, 2 g. ferrous sulfate 1, 468 1, 475

1 pH adjusted to about 5.5 with NaOH before adding reducing agent.

15 grams of a PdCl solution containing 26% Pd by analysis were mixedwith 85 grams of an AuCl solution containing 39.08% Au so that theweight of palladium in the resulting solution equaled 10% of the totalweight The observed melting point values demonstrate that the powderproducts formed are true alloys since their melting points are within 35C. of the theoretical values of the alloys. The observed melting pointsof the alloys are higher than 1062 C., the melting point of gold,further indicating that alloy particles and not mixtures of goldparticles and palladium particles are formed.

Tables VI and VII set forth the particle size distribution of thepowders of Examples 4 and 18, respectively. These powders were typicalof average particle size and particle size distribution for the powdersof the examples of tables 1V and V. These particle size analyses wereobtained by microscopic study of enlarged electron photo micrographs ofthe respective powders.

TABLE VI Gold-platinum alloy powder Percentage of particles Patriclesize range, microns: within indicated size range 2 TABLE VIIGold-platinum alloy powder Percentage of particles Patricle size range,microns: within indicated size range -0.1 None 0.1-1.0 75 1.04.0 90 -5050-100 4 100 2 The average particle size was 0.2 micron.

The alloy powders of this invention are characterized in beingirregularly shaped, and having a small average size resulting in a highsurface area to mass ratio and excellent conductive properties. Byreason of the fact that 90% by count of the particles are within a closesmall size range, between 0.1 and 5.0 microns, settling and verticleclassification of the particles during application and firing of themetalizing compound are reduced. More uniform high quality fired-oncoatings can accordingly be produced with the metal powders of thisinvention. Average particle sizes of about 40 microns and smaller arenecessary to enable screen printing thereof with 325 mesh screens.

The term reducing agent as used throughout the specification and claimsis meant to include single substances which are capable of individuallyprecipitating all of the metal constituents of the alloy to be formed aswell as combinations of substances which together pre cipitate all ofthe metal constituents of the allo to be formed.

Parts, percentages and proportions as herein disclosed unless otherwisestated relates to parts, percentages and proportions by weight.

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited thereto except as set forth in the appended claims.

I claim:

1. The process for the production of a finely divided alloy consistingessentially of two noble metals, which metals form continuous series ofsolid solutions throughout their entire alloy composition range withoutthe formation of compounds or eutectics which comprises forming asolution of compounds of the two noble metal constituents of the alloyto be formed with the two noble metal constituents present inapproximately the same relative proportions that they are to be presentin the alloy to be formed and with each of said two noble metalconstituents constituting from 10-90% of the total amount of said twonoble metal constituents, and mixing with said solution a reducing agentcapable of simultaneously reducing the metal constituents of thecompounds to their metals, whereby to precipitate alloy particles fromthe solution.

2. The process for the production of noble metal alloys consistingessentially of two noble metals selected from the group consisting ofsilver, gold, platinum and palladium, which comprises preparing asolution of compounds of the two noble metal constituents of the noblemetal alloy to be formed and with each of said two noble metalconstituents constituting from 10-90% of the total amount of said twonoble metal constituents, and mixing with said solution a reducing agentcapable of simultaneously reducing the metal constituents of thecompounds to their metals, whereby to precipitate alloy particles fromthe solution.

3. The process of claim 2 wherein the solution comprises dissolvedcompounds of platinum and gold.

4. The process of claim 3 wherein the reducing agent is hydrazinehydrate.

5. The process of claim 2 wherein the solution comprises dissolvedcompounds of palladium and gold.

6. The process of claim 5 wherein the reducing agent is hydrazinehydrate.

7. The process of claim 2 wherein the solution comprises dissolvedcompounds of palladium and silver.

8. The process of claim 7 wherein the reducing agent is hypophosphorousacid.

9. The process of claim 2 wherein the solution comprises dissolvedcompounds of silver and gold.

10. The process of claim 2 wherein the solution comprises dissolvedcompounds of platinum and palladium.

11. The process for the production of palladium-silver alloy whichcomprises preparing an acidic solution containing silver and palladiumnitrates, and precipitating a finely divided alloy of Pd-Ag by theaddition to said solution of a reducing agent that simultaneouslyreduces the silver and palladium nitrates to metal.

12. The process which comprises dissolving palladium and silver inconcentrated nitric acid containing at least 3% excess N0 to form asolution of palladium and silver nitrates, adjusting the pH of thesolution to between 4.5 and 6.5, and adding to said solution a reducingagent capable of simultaneously reducing said nitrates to their metals,whereby to precipitate Pd-AG alloy particles from the solution.

13. The process of claim 12 in which the reducing agent ishypophosphorous acid.

14. The process for the production of palladium-silver alloy whichcomprises dissolving palladium in concentrated nitric acid containing atleast 3% excess N0 dissolving silver in said nitric acid, adding to theresulting nitric acid solution of palladium and silver nitrates areducing agent capable of simultaneously reducing said palladium andsilver nitrates to their metals.

15. The process which comprises dissolving palladium and silver inconcentrated nitric acid containing at least 3% excess N0 to form anitric acid solution of palladium and silver nitrates, and adding tosaid solution hypophos phorous acid whereby to precipitate Pd-Ag alloyparticles from said solution.

16. The process of adding hypophosphorous acid to an acidic solution ofpalladium and silver nitrates whereby to precipitate from said solutionfinely divided Pd-Ag alloy particles containing palladium and silver inthe same relative proportions in which they were present in saidsolution.

17. The process of claim 2 wherein the solution comprises dissolvedcompounds of platinum and silver.

References Cited UNITED STATES PATENTS 12/1915 Sulzberger -108 8/1922Sulzberber 75-108 Disclaimer 3,390,981.Lewis C. H ofl'ma'n, W'ilmington,Del. METHOD FOR THE PRO- DUCTION OF FINELY DIVIDED METALS. Patent datedJuly 2, 1968. Disclaimer filed Apr. 30, 1976, by the assignee, E. I. duPont de 1V emours and Company. Hereby enters this disclaimer to claims1-7, 9-11, and 17 of said patent.

[Oflicial Gazette July 6', 1.976.]

1. THE PROCESS FOR THE PRODUCTION OF A FINELY DIVIDED ALLOY CONSISTINGESSENTIALLY OF TWO NOBLE METALS, WHICH METALS FROM CONTINUOUS SERIES OFSOLID SOLUTION THROUGHOUT THEIR ENTIRE ALLOY COMPOSITION RANGE WITHOUTTHE FORMATION OF COMPOUNDS OF EUTECTICS WHICH COMPRISES FORMING ASOLUTION OF COMPOUNDS OF THE TWO NOBLE METAL CONSTITUENTS OF THE ALLOYTO BE FORMED WITH THE TWO NOBLE METAL CONSTITUENTS PRESENT INAPPROXIMATELY THE SAME RELATIVE PROPORTIONS THAT THEY ARE TO PRESENT INTHE ALLOY TO BE FORMED AND WITH EACH OF SID TWO NOBLE METAL CONSTITUENTSCONSTITUTING FROM 10-90% OF THE TOTAL AMOUNT OF SAAID TWO NOBLE METALCONSTITUENTS, AND MIXING WITH SAID SOLUTION A REDUCING AGENT CAPABLE OFSIMULTANEOUSLY REDUCING THE METAL CONSTITUENTS OF THE COMPOUNDS TO THEIRMETALS, WHEREBY TO PRECIPITATE ALLOY PARTICLES FROM THE SOLUTION.